1. Field of the Invention
The present invention relates to an apparatus for comminuting material having a cool air channel.
2. Description of the Background Art
During the comminuting of materials in conventional devices, a considerable part of the energy required for the comminuting is converted into heat. This is caused by friction and impact forces the materials are subjected to during the comminuting process, and which primarily affect the comminuting tools.
A characteristic of conventional devices during operation is air flow, which, apart from the centrifugal force, is the force that moves the materials. This so-called self-ventilation can be generated by the device itself and/or initiated from the outside. If the material is not heat-sensitive, the innate self-generated flow of air in conventional devices is sufficient to cool down the comminuting tools such that any adverse effects on the material are eliminated.
Problems occur on a regular basis, when heat-sensitive materials are to be comminuted. Especially when plastics with a low melting point are to be comminuted, operators of conventional devices face a difficult task. On the one hand, the milling of the material is to be done at barely below the melting point in order to attain as high a machine output as possible. If the material-dependent temperature limit is thereby exceeded, the material softens and begins to melt with the result that individual particles bake together such that the size of the particles and the particle distribution of the milled material are no longer within the desired range. On the other hand, the overheated particles bake onto the machine parts, particularly the milling tools, so that the machine efficiency as well as the quality of the finished product leave much to be desired.
This problem is compounded when fine-milling heat-sensitive materials because it was found that the finer the finished product is to be, the more comminuting work has to be done, and the greater the heat generation in the area of the comminuting tools will be.
To avoid thermal overstress of the material during the comminuting process, it is common to lower the machine output of comminuting devices. In this way, less comminuting work is done per unit of time, thus generating less excess heat. However, as a consequence, the comminuting apparatus does not operate at full capacity, which goes against the fundamental requirements for an economical operation of such devices. One conventional solution is to increase the air volume beyond the self-ventilation of a conventional comminuting device by adding blowers in order to be able to vent additional heat.
Moreover, a device is known from DE 360 295 A1 having two axially spaced milling disks for forming a milling gap. The disk on the material-intake side is rigidly attached to the housing and is provided with openings for feeding the material into the device. The rear disk is positioned on a drive shaft to execute a rotational movement. For additional cooling, the rear disk is thick-walled and provided with a hollow space. The drive shaft, which is formed as a hollow shaft, has two channels, one of which feeds cooling fluids into the hollow space, whereas the other serves as the return line for the cooling fluids from the hollow space.
From U.S. Pat. No. 3,302,893, a device is known, wherein two axially opposed and rotating milling disks form a radial milling gap. The disk at the material-intake side has two openings to feed the material to the milling gap. The drive shaft for the rear milling disk is a hollow shaft for forming a cooling channel, from which cooling lines that are arranged in a star-shaped pattern in the area of the rear disk, lead to the comminuting tools. Cooling fluids from the cooling lines are directed to the area of the comminuting tools.
An additional device of this class is disclosed in U.S. Pat. No. 3,584,799. A milling disc rotates opposite a stationary milling ring, which is arranged at the intake side of the housing. The stationary milling ring is cooled with cool water, which is introduced via an annular groove in the housing, and distributed.
The disadvantage of this device is the need to provide a further cooling medium in addition to air. The additional technical expenditure necessary for storing, cooling, and conveying the cooling medium makes this device costly, in regard to both acquisition and operation.